The present invention relates to a process for carbonation of a living diene polymer obtained anionically. More precisely, the invention applies to the carbonation of a living polymer, at least one end of the molecule of which bears a C-M group, M being a metal, and in particular Li.
It is known that conjugated dienes and vinyl aromatic compounds may be polymerized or copolymerized with each other anionically, i.e., in the presence of an organometallic initiator, most frequently an organolithiated initiator. The process produces living polymers, having a C-M group or groups at the end(s) of the chain that can be used for a subsequent carbonation reaction.
Persons skilled in the art know that, during carbonation of a living polymer, the functionalization reaction which takes place produces not only a mono-adduct polymer having a carboxylic acid function at the chain end, (P)CO2H, P being a polymer chain, but also secondary products: a bi-adduct, (P)2CO, and a tri-adduct, (P)3OH.
Numerous methods have already been proposed to direct the carbonation reaction towards forming the mono-adduct and to avoid forming the bi- and tri-adducts.
For example, as described in the Exxon Chemical patent specification WO-A-91104 993, it is possible to react the carbon-metal chain end with a deactivating agent, such as 1,1-diphenylethylene, or alternatively a-methylstyrene, then to effect carbonation in the presence of a polar diluent (generally THF), at a temperature not exceeding 20xc2x0 C. (typically of the order of xe2x88x9210xc2x0 C.).
Another method described by R. P. Quirk and Tian Yin, Macromolecules, vol. 22, p. 85 (1989), involves reacting the solution of living polymer with carbon dioxide, which is solid, at a temperature which is even lower (of the order of xe2x88x9278xc2x0 C.), because it is known that the proportion of bi/tri-adducts is reduced by lowering the temperature.
Furthermore, it is generally accepted that the mixing conditions are a determining factor for controlling the proportion of the bi-/tri-adducts (see J. Villermaux, Genie de la Reaction Chimique, Conception et Fonctionnement des Reacteurs, Collection Technique et Documentation, Edition Lavoisier (2nd edition 1985) chapter 8).
From this perspective, it is known from patent specification GB-A-921 803 to effect the carbonation reaction of the living polymer in a zone of turbulence, for example in a T-shaped tube, into which zone a solution of living polymer and carbon dioxide is introduced.
In this process, a very great excess of carbon dioxide is used, representing 2 to 50 times the stoichiometrically necessary quantity. The reaction is carried out at a temperature of between xe2x88x9250xc2x0 C. and 15xc2x0 C., with the cold temperature necessary for cooling the solution of living polymer being obtained, due to the expansion of the carbon dioxide. The latter is introduced into a turbulence chamber by means of an injector at a pressure of 0.7 to 7xc3x97105 Pa.
East German patent specifications DD-A-139 661 and DD-A236 537 provide processes for the carbonation of living polymers which essentially consist of spraying a living polymer, for example a lithiated one, into a turbulent carbon dioxide current characterized by a very high pressure and flow rate. The pressure is set at 8xc3x97105 Pa, and said flow rate is at least 2,500 l/h. The carbonation temperature must be kept within a range from xe2x88x9220xc2x0 C. to 20xc2x0 C. approximately, and is preferably kept at 0xc2x0 C.
One major disadvantage of these processes using carbon dioxide under turbulent conditions is that they require a large amount of this gas, one significantly greater than the stoichiometric quantity. Because of this, the implementation of a recycling system and purification of the excess carbon dioxide may be necessary, owing to the production and tonnage constraints which are required on an industrial scale.
Another disadvantage of these processes resides in the necessity of controlling the temperature of the reaction medium or of the incoming flows, such that the temperature of said reaction medium is permanently located in a restricted range limited to relatively low temperatures, in order to permit a good carboxylation yield.
U.S. Pat. No. 3,227,701 discloses, in its examples, a carbonation process which involves spraying a living polybutadiene into a chamber and into contact with carbon dioxide at rest. The polymer is brought into the chamber under pressure by means of a dispersion device comprising, at its outlet, a motor-driven rotary disc having deflectors surrounding said disc which are intended to divide the particles of polybutadiene dispersed into the chamber thereby.
The process described in the xe2x80x98701 patent provides that the temperature of the carbonation reaction lies between approximately xe2x88x9250 and 15xc2x0 C., and the pressure of carbon dioxide in the chamber varies substantially from 105 to 10xc3x97105 Pa. One major disadvantage of this process lies in the relatively narrow temperature and pressure ranges which can be used.
The inventors have discovered an improved process for carbonation of a living diene polymer which involves spraying a solution of the polymer, in the form of droplets, through one (or more) nozzle(s) into a chamber containing a carbon dioxide bath substantially at rest which fills said chamber. Surprisingly, a difference in hydrostatic pressure between the inlet to the nozzle and the chamber, which allows the sprayed solution and carbon dioxide bath to interact for carbonation of the polymer, overcomes the disadvantages of prior art processes and provides for obtaining a high yield of mono-adduct. The difference in hydrostatic pressure achieves a particular division of the sprayed particles of living polymer solution, making it possible to carry out the carbonation reaction at a wider temperature range, advantageously between xe2x88x9250xc2x0 C. and 100xc2x0 C.